Temperature sensors are utilized in a variety of applications. For example, temperature sensors that are used in conjunction with ovens typically comprise a metallic tube in which a temperature sensitive element is disposed inside one end with conductive wires extending within the tube from the temperature sensitive element to an opening at the other end of the tube. The metallic tube is inserted through a wall of the oven to permit the temperature sensitive element to be placed in thermal communication with the internal cavity of the oven. The temperature sensitive element is typically a resistive temperature detector, or RTD. The temperature sensor can be also based on a thermistor or thermocouple configuration, a metal oxide semiconductor, or any other type of temperature sensing element.
One area where temperature sensors find particular usefulness is in the area of exhaust gas environments. Various applications require measurement of temperature of gas or mixture of gases at elevated temperatures. One such application involves automotive or combustion applications in which a need exists for measuring the exhaust gas temperature for emission control using Selective catalytic reduction (SCR) and Exhaust Gas Recirculation (EGR) based emission after treatment systems. The sensor should function in a harsh and corrosive automotive exhaust gas environment containing, for example, soot particles, SOx, moisture, diesel, NH3, NOx, HC, CO, CO2 etc.
Exhaust gas temperature (EGT) can be utilized to measure the performance, for example, of an automotive engine. The exhaust gas temperature also provides an indication of the rate of deterioration of automotive engine components. Thus, since the exhaust gas temperature is an indicator of engine status, it may be used to measure and control operational and functional characteristics of the engine.
Accurate measurement of the exhaust gas temperature level is important. To accurately measure exhaust gas temperatures, it is necessary to minimize degradation of the EGT measurement system. Thus it is desirable that the EGT measurement system compensate for engine to engine variations and combustor exit temperature profiles. In addition, the measurement system should compensate for shifts in engine profiles that may occur with progressive deterioration of the engine components.
The penetration of a particular sensor can be determined by the temperature profile of the exhaust gases. The exhaust gas temperature profile is determined by the number, type and arrangement of the combustion nozzles in the combustor. The exhaust gas temperature profile for a particular engine may be determined by using a large number of thermocouple elements arranged in a number of sensors around the exhaust passage and at various penetration depths. Once the exhaust gas temperature profile is defined for a particular type of engine, it may be used to calculate the number and arrangement of EGT sensors necessary to monitor the exhaust gas temperature during normal engine operation.
As indicated above, a variety of temperature sensing elements can be utilized in the context of an exhaust gas temperature sensor. Resistance Temperature detectors (RTD) elements can be used in temperature measuring equipment. The RTD Element has a ceramic substrate with a platinum or nickel or similar metal thin/thick film resistor with an over coating of a protective layer like glass or ceramic or any other material glazing, which is thermally a good conductor. Wire wound RTD elements are also available. Materials such as, for example, platinum or nickel have a positive co-efficient of temperature and the resistance increases linearly with increase in temperature.
Thermistors are also utilized in temperature measuring equipment. Thermistors are essentially semiconductor devices, which behave as thermal resistors having high negative or positive temperature co-efficient of resistance. Thermistors are made of sintered metal oxide ceramics like oxides of iron, magnesium, nickel, cobalt and copper in the form of beads or discs or rods. The variation in temperature is non linear, resistance decreases with increase in temperature in case of negative temperature co-efficient (NTC) of resistance thermistor and resistance increases with increase in temperature in case of positive temperature co-efficient (PTC) of resistance thermistor.
Thermocouples, for example, are the most commonly used temperature sensing devices and operated based on the principle of the so-called See-Beck effect, i.e., when two dissimilar metal or ceramic or metal oxide semiconductor junctions are maintained at different temperature an EMF is induced at the junction, which is proportional to temperature difference. Generally Platinum with copper, Constantan, Nickel, Rhodium, Iron, Gold, ZrO2, Al2O3, CeO2 and so forth can be utilized.
The sensing element can be suitably packaged and placed in a gas flow path and the temperature is measured by using a suitable electronic circuit by transduction of resistance or voltage. In general, two types of packaging methods are available. The first method involves the so-called Open Sensing Tip Sensor method where the sensing element is exposed to gas stream directly. The second method is the Closed Sensing Tip Sensor method, where the sensing element is not directly exposed to gas stream and packaged inside a metal housing high temperature potting around the sensing element. These techniques have specific disadvantages, which are summarized below in Table 1:
TABLE 1Open Sensing Tip SensorClosed Sensing Tip SensorDisadvantages1. Has less protection for sensing1. The Closed Sensing Tip  element from NH3/SOx/NOx/  construction has slower  moisture/Diesel/SO2/soot  response (t63) as the sensing  particles etc available in the  element is not directly exposed  harsh exhaust environment.  to gas.2. Has less protection for sensing2. Less accurate at high  element against vibration and  temperatures as there is  shock.  significant temperature gradient3. Has less leak protection at the  between gas and sensing  sensing tip.  element.4. Has less reliability because of  the above reasons.
The ability to accurately measure temperature in an exhaust gas environment can be based on a number of critical factors. For example, in some applications it is desirable to achieve a dynamic response time t63 less than 15 seconds and a static response for an RTD-based closed sensor intended for measuring the temperature of exhaust gas in an automotive environment. The dynamic response t63 represents the time taken by the sensor to attain 63.2% of the medium temperature for a particular test condition (e.g., medium, temperature of the medium, flow rate of the medium, density of the medium, etc). Static response, on the other hand, constitutes the temperature difference between the outer surface of the metal housing and RTD surface under a static steady state condition and can have a significant impact on the accuracy of the sensor. It is preferred that such a closed sensor be subjected to harsh hot exhaust gas, along with, for example, diesel, moisture, vibration and/or other fluids and/or gasses over a temperature range of, for example, −40° C. to 750° C.
Prior art approaches involve utilizing a small tip structure to house an RTD sensing element. Such devices, however, have not been found to work in such a scenario because of the physical limitations of mass-to-area ratio (m/a), which governs the response time. Thus, there is a need to reduce the m/a ratio in order to reduce the response time in order to achieve greater efficiencies and accurate readings in a gas exhaust environment utilizing an RTD-based temperature sensor. It is believed that the system and method disclosed herein in greater detail solves this important and heretofore unmet need.